1. Field of the Invention
The present invention relates to a position detection magnet that is provided on a yoke, and a position detection apparatus that uses the position detection magnet for detection.
2. Description of the Related Art
A shifting device that utilizes a position detection magnet for vehicular application is known. An example of such a vehicular shifting device is disclosed in Japanese Unexamined Patent Application Publication No. 2007-223384.
Though a magnetic-type position detection apparatus, which uses a magnet for positional detection, is used not only in a vehicular shifting device but also in various devices such as a joystick input device, a sliding-type four direction switch, and the like, a position detection apparatus with a position detection magnet for vehicular shifting application is explained below with reference to FIG. 11. A shift lever 1 can be moved with a shift movement support member 2 as a fulcrum of the movement. A magnet holder 3 is fixed to the front end of the shift lever 1. A gearshift knob 4, which is operated by a driver, is fixed to the other opposite end of the shift lever 1. A cover plate 5 has a guiding opening that is formed along the shift movement paths of the shift lever 1. A magnet (denoted as 16 in related art and as 6 according to an exemplary embodiment of the present invention) that has the shape of a flat plate is attached to the magnet holder 3. A substrate 7 is provided opposite the magnet 16 with a predetermined clearance between the magnet 16 and the substrate 7. A plurality of magnetic detection elements 8 is provided on the substrate 7. A magnet movement mechanism that is not illustrated in the drawing enables the magnet 16 to move on a virtual plane that is parallel to the surface of the substrate 7. When a driver operates the gearshift knob 4, the position of the magnet 16 with respect to the magnetic detection elements 8 (such as a Hall element or the like) changes in synchronization with the movement position of the gearshift knob 4. Due to the change in the position of the magnet 16, the magnetic detection state of each of the magnetic detection elements 8, which detect the magnet 16, also changes. As a result, a position detection signal in accordance with the detection state is outputted. The operation position of the gearshift knob 4 is judged on the basis of the position detection signal. Then, a resultant shift signal is outputted to a gear-changing device, etc. of the vehicle.
A magnet of related art used in the position detection apparatus explained above has a structure illustrated in FIG. 12. An yoke 19 that has a bent shape to surround the sides of the magnet 16 is bonded to the reverse face of the magnet 16. The magnet 16 has the shape of a flat plate. The position of the related-art magnet (16) illustrated in FIG. 12 is indirectly determined by means of side parts 19a of the yoke 19 when fixed to the magnet holder 3. Therefore, in the position detection apparatus having the above structure, the positional precision of the magnet 16 with respect to the yoke 19 greatly affects the detection accuracy of the position detection apparatus as a whole. The reason why the detection accuracy of the entire position detection apparatus is greatly affected thereby is explained below.
The yoke 19 illustrated in FIG. 12 has a base part 19b in addition to the side parts 19a. As illustrated in FIG. 13, when the structure of the magnet 16 and the yoke 19 illustrated in FIG. 12 is adopted, magnetic lines of force formed by the magnet 16 converge at the side parts 19a of the yoke 19 because of the presence of the side parts 19a and the base part 19b. If the yoke 19 were not attached to the magnet 16, magnetic lines of force (magnetic field) that exit from the surface (N-pole surface) of the magnet 16 would take a circuitous route in the air over the side surfaces of the magnet 16 to enter the back (S-pole surface) thereof. However, since the yoke 19 having high magnetic permeability is provided, at the back of the magnet 16, the base part 19b of the yoke 19 shuts off magnetic lines of force coming toward the S poles. In addition, a magnetic path that guides magnetic lines of force is formed in the side parts 19a and the base part 19b. Therefore, magnetic lines of force flowing out from the surface of the magnet 16 converge at the side parts 19a of the yoke 19, which are close to the N poles of the magnet 16. Then, the magnetic lines of force enter the S poles of the magnet 16 through the base part 19b of the yoke 19.
Therefore, when the magnet 16 is used in a position detection apparatus, the borderline of the output level 0V of a magnetic detection element such as a Hall element or the like extends from the border between magnet 16 and the side part 19a of the yoke 19 as a line (or a plane if viewed in three dimensions) that is, roughly speaking, perpendicular to the surface of the magnet 16 as illustrated in the drawing. The borderline of the output level 0V of a magnetic detection element such as a Hall element is a border at which the detection state of the magnetic detection element changes over from “detected” to “undetected”. Since the borderline of the output level 0V is roughly perpendicular thereto, a change in the detection state of the magnetic detection element in response to the movement of the magnet 16 in directions including X direction is very sensitive. Therefore, high position detection precision is ensured. In the drawing, the borderline of the Hall output level 0V means the output level 0V of the Hall element as the magnetic detection element.
The yoke 19 and the magnet 16 are bonded to each other to make up a single bonded member. It is inevitable to design a slight dimension difference between the X-directional width of the magnet 16 and the X-directional inner distance between one of the side parts 19a of the yoke 19 and the other as a margin for the purpose of fixing the magnet 16 between the side parts 19a of the yoke 19 easily. That is, a clearance is necessary for the manufacturing reason. Due to the presence of the clearance, when the yoke 19 and the magnet 16 are bonded to each other, the X-directional position of the magnet 16 with respect to the yoke 19 will be shifted from the supposed position. This means that a gap between the side part 19a of the yoke 19 and the side part of the magnet 16 varies. In addition, the erection dimension (in Z direction) of the side part 19a varies due to limited machining accuracy in the working of the side part 19a of the yoke 19. Moreover, the amount of an adhesive used for the bonding of the yoke 19 and the magnet 16 also varies. For these reasons, the relative positions of the side parts 19a of the yoke 19 and the side parts of the magnet 16 vary not only in the X direction but also in the Z direction. The positional relationship varies from one bonded piece made up of the magnet 16 and the yoke 19 to another as individual differences. In addition to the individual differences, the positional relationship varies from one magnet-peripheral area to another even in each single bonded member made up of the magnet 16 and the yoke 19. The variation explained above affects the X-directional position of the borderline of the Hall output level 0V and, in addition, affects perpendicularity to the surface of the magnet 16.
The magnet 16 that is bonded to the yoke 19 to make up a single bonded member is mounted onto the magnet holder 3 through the fixation of the yoke 19 to the magnet holder 3. When the yoke 19 is fixed to the magnet holder 3, the yoke 19 is subjected to positioning. However, because of variation that occurs in the process of bonding the yoke 19 and the magnet 16 to each other as explained above, the position of the borderline of the Hall output level 0V varies. For this reason, even in a case where the yoke 19 is fixed to the magnet holder 3 at an accurate position, the borderline of the Hall output level 0V varies with respect to the magnet holder 3. The position detection apparatus explained above detects the operation position of the gearshift knob 4 on the basis of the detected position of the magnet 16 mounted to the magnet holder 3. Therefore, if the borderline of the Hall output level 0V varies with respect to the magnet holder 3, the detection accuracy of the position detection apparatus decreases.